High-Pressure Nmr And Compressibility Evidence For A Phase-Transition In The Protonic Conductor (NH4)4Fe(CN)6.1.5H2o

1H NMR at high hydrostatic pressures and compressibility studies show that the protonic conductor (NH4)4Fe(CN)6·1.5H2O undergoes a phase transition around 0.45 GPa. The transition is characterized by a large hysteresis. From the NMR studies, an activation volume of 6% is obtained below the phase transition, indicating the dominance of Frenkel defects.

High-pressure NMR investigations of the protonic conductor (NH4)4Fe(CN)6.1.5H2O

The effect of high hydrostatic pressure up to 1.5 GPa on ionic motion in (NH4)4Fe(CN)6.1.5H2O has been studied by wide-line 1H NMR experiments performed in the temperature range from room temperature to 77 K. The experiments at room temperature have shown a large increase in the second moment at 0.45 GPa as a result of a pressure-induced phase transition. The temperature dependence study up to 0.425 GPa has shown a gradual increase in the values of activation energy and attempt frequency with increase in pressure. The activation volume for motion at 300 K has been estimated to be 6% of molar volume. Vacancy-assisted ionic jumps are concluded to be the mode of charge transport. Second moments estimated at 77 K show evidence for tunnelling reorientation of at least one of the two NH4+ groups in the compound.

NMR and conductivity study of the protonic conductor HPb2Nb3O10 center dot nH(2)O

Electrical conductivity and H-1 Nuclear Magnetic Resonance (NMR) techniques were used to investigate the ion-exchanged layered lead-niobate perovskite HPb2Nb3O10. nH(2)O, over the temperature range 90-350 K. Compounds were synthesized by the sol-gel method and calcinated at 650 degreesC. Analysis of the NMR data gives activation energies for the proton motion in the range 0.14-0.40 eV, which are dependent on the water content. The frequency and temperature dependencies of the proton spin-lattice relaxation times show that the character of the motion of the: water molecules is essentially two-dimensional, reflecting the layered structure of the material. The H-1 line-narrowing transition and the single spin-lattice relaxation rate maximum, observed in the hydrated compounds, are consistent with a Grotthuss-like mechanism for the proton diffusion. (C) 2000 Elsevier B.V. B.V. All rights reserved.

Evaluation Of Diffusion-Coefficient And Ionic Mobility In (Nh4)4fe(Cn)6.1.5 H2o

The diffusion coefficient, D, and the ionic mobility, μ, in the protonic conductor ammonium ferrocyanide hydrate have been determined by the isothermal transient ionic current method. D is also determined from the time dependence of the build up of potential across the samples and theretical expressions describing this build up in terms of double exponential dependence on time are obtained. The values obtained are D=3.875×10−11m2s−1 and μ=1.65×10−9 m2V−1s−1.

Concentration-dependent NMR and conductivity studies of (PEG)(x)NH4ClO4

Films of (PEG)(x)NH4ClO4 (x = 5 to 1000) were prepared and characterized. The physical properties are observed to be a sensitive function of concentration. Hygroscopicity increases as salt content increases. Conductivity peaks (sigma = 2.7 x 10(-6) S/cm) at x = 46. The H-1 NMR line width has a minimum at x = 46, while that of Cl-35 monotonically increases with salt concentration, indicating that the complex is essentially a protonic conductor.

Influence of the indium concentration on microstructural and electrical properties of proton conducting NiO-BaCe0.9-xIn xY0.1O3-δ cermet anodes for IT-SOFC application

Optimization of the major properties of anodes based on proton conductors, such as microstructure, conductivity and chemical stability, is yet to be achieved. In this study we investigated the influence of indium on the chemical stability, microstructural and electrical characteristics of proton conducting NiO-BaCe0.9-xInxY0.1O 3-δ (NiO-BCIYx) anodes. Four compositions of cermet anode substrates NiO-BCIYx were prepared using the method of evaporation and decomposition of solutions and suspensions (EDSS). Sintered anode substrates were reduced and their microstructural and electrical properties were examined before and after reduction as a function of the amount of indium. Anode substrates tested on chemical stability in the CO2 atmosphere showed high stability compared to anode substrates based on commonly used doped barium cerates. Microstructural properties of the anode pellets before and after testing in CO2 were investigated using X-ray diffraction analysis. Impedance spectroscopy measurements were used for evaluation of electrical properties of the anode pellets and the conductivity values of reduced anodes of more than 14 S cm-1 at 600 °C confirmed percolations through Ni particles. Under fuel cell operating conditions, the cell with a Ni-BCIY20 anode achieved the highest performance, demonstrating a peak power density 223 mW/cm2 at 700 °C confirming the functionality of Ni-BCIY anodes.© 2013 Elsevier B.V. All rights reserved.

Stability, characterization and functionality of proton conducting NiO-BaCe0.85-xNbxY0.15O3-delta cermet anodes for IT-SOFC application

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Enhancement of Electrical Conductivity in Plasticized Chitosan Based Membranes

Plasticized natural macromolecules-based polymer electrolyte samples were prepared and characterized. The plasticization of chitosonium acetate with glycerol increased the ionic conductivity value from 3.0 x 10(-7) S/cm to 1.1 x 10(-5) S/cm. The conductivity temperature relationship of the samples exhibits either VTF or Arrhenius type depending on the glycerol concentration in the sample. The dielectric studies evidencing the relaxation process in the plasticized sample at low frequency region are due to the electric polarization effect. Moreover, the samples were transparent in the Vis region, showed thermal stability up to 160 degrees C and good surface uniformity.

Densification and enhancement of the grain boundary conductivity of gadolinium-doped barium cerate by ultra fast flash grain welding

Doped barium cerate is a promising solid electrolyte for intermediate temperature fuel cells as a protonic conductor. However, it is difficult to sinter it to high density at a reasonable temperature. Moreover, it presents a high grain boundary resistivity at intermediate temperatures. Flash grain welding was applied to compacted samples, starting from a temperature of 910 degrees C and applying, for a short time, an ac electric polarization of 40 V, 1000 Hz. At that frequency, the resulting current flows through the grain boundaries promoting a welding via a local Joule heating. A large decrease of the grain boundary resistivity was observed by impedance spectroscopy. Scanning electron microscopy observations of polished and etched surfaces revealed highly sintered regions. Attempts were also made to combine flash grain welding with conventional sintering. (C) 2012 Elsevier Ltd. All rights reserved.

Wireless electrochemical modification of catalytic activity on a mixed protonic-electronic conductor

A novel approach to electrochemical modification of catalytic activity using a wireless configuration has been undertaken. This paper presents preliminary results on the modification of a platinum catalyst film supported on a pellet of Sr_0.97Ce_0.9Yb_0.1O_3-δ (SCYb), considered to be a mixed protonic-electronic conductor under reducing conditions. The wireless configuration utilises the mixed ionic and electronic conductivity of the supporting membrane to supply an ionic promoting species to the catalyst surface. Control of the flux of this species is achieved by adjusting the effective hydrogen chemical potential difference across the membrane in a dual-chamber reactor with one chamber acting as the "reaction side" and the other as the "sweep side". The reaction rate can be promoted by up to a factor of 1.6, for temperatures around 500 °C and low reactant concentrations, when hydrogen is introduced on the sweep side of the membrane reactor. The use of helium, moist helium and oxygen in helium as sweep gases did not modify the reaction rate. © 2007 Elsevier B.V. All rights reserved.

(PEG)(x)NH4ClO4: A new polymeric fast proton conductor

A new polymer electrolyte (PEG)(x) NH4ClO4(x = 5, 10, 15, 20) has been prepared that shows protonic conduction. The room temperature conductivities are of the order of 10(-7) S/cm, and increase with decrease in salt concentration. NMR line width studies indicate fairly low glass transition temperatures of the polymer salt complexes.

Electrochemical promotion of a platinum catalyst supported on the high-temperature proton conductor La0.99Sr0.01NbO4-δ

The recently discovered, high-temperature proton conductor, La_0.99Sr_0.01NbO_4-δ, was used as a support for the electrochemical promotion of a platinum catalyst. Ethylene oxidation was used as a probe reaction in the temperature range 350-450 °C. Moderate non-Faradaic rate modification, attributable to a protonic promoting species, occurred under negative polarisation; some permanent promotion was also observed. In oxidative atmospheres, both the p_O2 of the reaction mixture and the temperature influenced the type and magnitude of the observed rate modification. Rate-enhancement values of up to ρ = 1.4 and Faradaic-efficiency values approaching Λ = -100 were obtained. Promotion was observed under positive polarisation and relatively dry, oxygen-rich atmospheres suggesting that some oxygen ion conductivity may occur under these conditions. Impedance spectroscopy performed in atmospheres of 4 kPa O₂/N₂ and of 5 kPa H₂/N₂ under dry and slightly humidified (0.3 kPa H₂O) conditions indicated that the electrical resistivity is heavily dominated by the grain-boundary response in the temperature range of the EPOC studies; much lower grain-boundary impedances in the wetter conditions are likely to be attributable to proton transport. © 2009 Elsevier B.V. All rights reserved.

Electrical relaxation in proton conductor composites based on (NH4)H2PO4/TiO2

We report on electrical relaxation measurements of (1-x)NH4H2PO4-xTiO(2) (x = 0.1) composites by admittance spectroscopy, in the 40-Hz-5-MHz frequency range and at temperatures between 303 and 563 K. Simultaneous thermal and electrical measurements on the composites identify a stable crystalline phase between 373 and 463 K. The real part of the conductivity, sigma', shows a power-law frequency dependence below 523 K, which is well described by Jonscher's expression sigma' = sigma(0)(1 + (omega/omega(p))(n)), where sigma(0) is the dc conductivity, omega(p)/2 pi = f(p) is a characteristic relaxation frequency, and n is a fractional exponent between 0 and 1. Both sigma(0) and f(p) are thermally activated with nearly the same activation energy in the II region, indicating that the dispersive conductivity originates from the migration of protons. However, activation energies decrease from 0.55 to 0.35 eV and n increases toward 1.0, as the concentration of TiO2 nanoparticles increases, thus, enhancing cooperative correlation among moving ions. The highest dc conductivity is obtained for the composite x = 0.05 concentration, with values above room temperature about three orders of magnitude higher than that of crystalline NH4H2PO4 (ADP), reaching values on the order of 0.1 (Omega cm)(-1) above 543 K.

Enhanced earth grid performance by improved conductor properties

Encroaching built environment with increased fault current levels is demanding a robust design approach and prolonged improved performance of the earth grid. With this in mind, the aim of the project was to perform a sensitivity analysis of the earth grid and an earthing performance evaluation with graphene coated conductors. Subsequent to these, a conceptual design to continuously monitor the performance of the earth grid was developed. In this study, earth grid design standards were compared to evaluate their appropriate use in determining the safety condition. A process to grow a thin film of graphene on the surface of cylindrical copper rods was developed to evaluate earthing performance in terms of conductivity and corrosion susceptibility.